This present invention relates to reducing the effect of narrowband noise in a multi-carrier transmission system.
In today""s modern world, businesses and residential users are demanding faster network access to the Internet. The high demand for faster network access is putting pressure on vendors and service providers to choose network transmission technologies that will satisfy the emerging demand. The choice of network transmission technologies is critical since it may affect service, cost, and ultimately vendor/service provider success.
Many of the vendors and service providers have chosen to pursue digital subscriber line (DSL) technology and more specifically asymmetrical DSL (ADSL) for providing fast Internet access to business and residential users. ADSL often provides high-speed data transmission over standard telephone lines while maintaining voice traffic on the same lines. ADSL may be seen as a cost-effective alternative to other network transmission technologies.
ADSL technology often exploits the relatively high bandwidth of copper loops by converting twisted-pair copper telephone wires into paths for multimedia, data communications, and Internet access. Typically, ADSL supports 1.544 to 6 Mbps transmission downstream and 640 kb/s upstream. ADSL service may be provided by connecting a pair of modems, one often located in the telephone company""s central office (CO) and the other located at the customer premises, over a standard telephone line.
An ADSL modem utilizing ANSI appointed discrete multitone (xe2x80x9cDMTxe2x80x9d) as the modulation scheme segment the frequency spectrum on a copper line into 256 channels. Each 4 kHz channel is capable of carrying up to 15 data bits according to the American National Standards Institute (ANSI) Standard T1.413, the contents of which are incorporated herein by reference. A similar standard, Recommendation G.992.1 from the ITU, is also incorporated herein by reference. A variation of the standard that accommodates POTS service without the use of a signal splitter is set forth in specification G.lite, or Recommendation G.992.2, the contents of which are incorporated herein by reference.
During channel analysis, a wide-band test signal sent over the 256 channels is transmitted from the ADSL terminal unit (xe2x80x9cATU-Cxe2x80x9d) at the CO to an ADSL remote terminal unit (xe2x80x9cATU-Rxe2x80x9d) at the customer premises. The ATU-R measures and updates the noise content of each of the channels received and then determines whether a channel has sufficient quality to be used for further transmission. Depending on the quality, the ATU-R may instruct the ATU-C how much data this channel should carry relative to the other channels that are used. Often, this procedure maximizes performance and minimizes error probability at any data specific rate. For instance, with a DMT modem, bit distribution may avoid noise by not loading bits onto channels that are corrupted by AM radio interference. The DMT modem may also lower bit distribution at the frequencies where notching occurs.
However, there are nearly 5,000 AM radio stations licensed in the U.S. to broadcast at frequencies between 540 kHz and 1.7 MHz. Unfortunately, ADSL service providers use the frequencies between 138 kHz and 1.1 MHz to download and upload data. This sizeable overlap (i.e., 560 kHz of bandwidth) can cause conflict because AM radio and ADSL try to use the same electromagnetic frequencies at the same time. Thus, as explained earlier, ADSL modems typically stop using the segment of the frequency spectrum occupied by any nearby AM stations. When an AM signal interferes with a carrier, a current remedy is to stop using that carrier and consequently reduces the bandwidth and data throughput.
Additionally, the longer a wire is from the central office to the remote terminal, the more susceptible the ADSL line is to interference as the signal gets weaker as it travels down the wire. The effect is particularly pronounced if the AM transmitter is near the remote terminal at the end of a long wire.
Interference caused from AM radio stations is part of a group commonly referred to as narrowband interference. Narrowband interference includes a signal whose essential spectral content may be contained within a voice channel on nominal 4-kHz bandwidth such as found in Amateur radio, AM, and FM radio signals. For instance, consider an AM transmission occurring at the frequency of 1070 kHz. If an ADSL signal is at the same frequency in a wire, then the ADSL receivers at the end of the wire may pick up the AM signal at 1070 kHz. To avoid this interference, data is simply not transmitted on that particular frequency thus also reducing possible throughput of the communication channel. This technique is currently used by the modulation standard of ADSL T1.413.
Thus, there is a need to reduce narrowband interference to increase throughput in a multi-carrier communications.
The system and method of the preferred embodiments may be directed to improving the signal-to-noise ratio in frequency spectrum regions where narrowband interference may be present. The system and method of the preferred embodiments includes reducing the narrowband interference by determining a noise estimate. In accordance with the noise estimate and output of a frequency domain equalizer, a noise-cancelled output may be obtained.
In accordance with one aspect of the present invention, a method for improving the signal-to-noise ratio in frequency spectrum regions where narrowband interference may be present includes the step of receiving at least one decoder error for the at least one carrier. Then, the step of determining at least one adaptive filter tap for each of the at least one carrier in relation to the received decoder error(s). The step of forming a noise estimate relating to the decoder error(s) and the adaptive filter tap(s). Then the step of receiving an FEQ output in relation with a frequency domain equalizer. Finally, the step of determining a signal having increased signal-to-noise ratio in response to the noise estimate and the FEQ output.
In accordance with another aspect of the present invention, a device for increasing a signal-to-noise ratio for at least one carrier in a multicarrier transceiver includes a canceller and a symbol storage unit. The canceller receives at least one decoder error for the at least one carrier and an FEQ output in relation with a frequency domain equalizer. The symbol storage unit stores the at least one decoder error. The canceller may then determine at least one adaptive filter tap for each of the at least one carrier in accordance with the at least one stored decoder error and forms a noise estimate relating to the at least one decoder error and the adaptive filter tap(s).
In a preferred embodiment, the reduction of narrowband interference is performed by a DMT receiver utilizing ADSL protocol. In another preferred embodiment, the receiver utilizes DSL protocol and any DSL variation protocol such as ADSL, very high data-rate DSL (xe2x80x9cVDSLxe2x80x9d), high bit-rate DSL (xe2x80x9cHDSLxe2x80x9d), and rate-adaptive DSL (xe2x80x9cRADSLxe2x80x9d).